Hood assembly utilizing active materials based mechanisms

ABSTRACT

An active material hood impact mitigation mechanism is activated in response to a signal generated from an impact sensor or pre-impact sensor or manually. The mitigation mechanism is capable of changing either reversibly or irreversibly the stiffness, shape, location, orientation, or displacement force of the hood either globally or locally, before an impact against the hood. The active material mitigation mechanism is held in a device designed to be installed in operative communication with the hood surface. The active material is characterized by a first shape or stiffness and is operative to change to a second shape or stiffness in response to the activation signal. Such active materials include shape memory alloys, electroactive polymers, shape memory polymers, magnetic shape memory alloys, magnetorheological fluids, magnetorheological elastomers, electrorheological fluids, and piezoelectric materials.

BACKGROUND

The present disclosure generally relates to a hood assembly for use inan automotive vehicle, wherein the hood assembly includes the use ofactive materials based mechanisms.

Numerous motor vehicles employ a hingeable hood disposed in a regionbetween the passenger compartment and the forward bumper of the motorvehicle, or between the passenger compartment and the rearward bumper ofthe motor vehicle. The hingeable hood provides a mechanism for accessingthe underlying engine or storage compartment. The vehicle hood istypically formed of a relatively thin sheet of metal or plastic that ismolded to the appropriate contour corresponding to the overall vehiclebody design. The exterior of the hood portion, which constitutes theshow surface thereof, is typically coated with one or more coats ofprimer and paint for enhancing both the aesthetic character and thecorrosion resistance of the underlying material. Due to the relativelythin nature of the material forming the hood portion, a supportstructure such as a contoured plate with stamped rib supports typicallyextends across the underside of the hood portion so as to provide adegree of dimensional stability to the structure.

Aerodynamics, styling, and packaging considerations, among others, haveall contributed to the design of the front ends and hood regions ofcurrent vehicles. Aerodynamic drag (and fuel economy considerations) inparticular has contributed to the hood being in close proximity to theengine or storage compartment. Accordingly, hood deformation such as mayoccur upon impact of an object onto the hood, and thus the ability ofthe hood to absorb energy at appropriate force levels before bottomingout against hard objects beneath it, is somewhat limited by the contentsof the compartment.

In response, automobile manufacturers have proposed a number ofmechanisms that change the orientation of the hood before a deformationevent such as the impact event previously described. For example, hoodlifters may be activated by impact sensors to increase the space betweenthe hood and the underlying compartment. The hood lifters change theorientation of the hood by raising it (in most mechanisms by raising itat a rear edge while maintaining attachment of a front edge to thevehicle structure, i.e., tilting) above the engine compartment. Upondeformation then, because of the increase in clearance there is anincrease in the amount of the energy that can be absorbed by deformationof the sheet metal before bottoming out. One drawback to such hoodlifting mechanisms is that they tend to be irreversible (which makesthem best suited for use only with crash and not with pre-crashsensors), so that such mechanisms will need to be replaced/repaired evenif collision does not in fact occur.

Accordingly, there remains a need in the art for automotive hoodcomponents having improved energy absorbing capabilities such as mayoccur upon deformation of the hood. The means/mechanisms that producethis energy absorbing capabilities are preferably reversible as well.

BRIEF SUMMARY

Disclosed herein are methods, devices, systems, and hingeable hoodassemblies utilizing an active material enabled approach to provideenhanced energy absorption properties. In one embodiment, the hoodassembly comprises a hood; an active material disposed in operativecommunication with the hood, wherein the active material comprises ashape memory alloy, a shape memory polymer, a magnetorheological fluid,an electroactive polymer, a magnetic shape memory alloy, amagnetorheological elastomer, an electrorheological fluid, apiezoelectric material, or combinations comprising at least one of theforegoing active materials; and an activation device coupled to theactive material, the activation device being operable to selectivelyprovide an activation signal to the active material and effectuate achange in the shape, dimension, and/or flexural modulus property (shearproperty, if the active material is liquid) of the active material,wherein the change in the shape, dimension, and/or flexural modulusproperties of the active material changes an energy absorption propertyof the hood.

A device positioned in operative communication with a hood hingeablyattached to a vehicle comprises an active material operative to changefrom a first shape, dimension, or a first stiffness to a second shape,dimension, or second stiffness in response to an activation signal,wherein the second shape, dimension, effectively increases the energyabsorbing properties of the hood.

A system for mitigating damage to an object from impact with a vehiclehood comprises a sensor that generates a signal based on pre-impact orimpact information; and a controller disposed to receive the sensorsignal and deliver an activation signal to at least one device inoperative communication with the hood; wherein the at least one devicecomprises an active material operative to change from a first shape, afirst dimension, or a first stiffness to a second shape, a seconddimension, or second stiffness in response to the activation signal.

A method for reducing damage to an object from an impact with a hoodhingeably attached to a vehicle comprises sensing the impact; generatingan activation signal; and activating a device in response to theactivation signal, wherein the device changes a position of the hoodfrom a first position to a second position by actively changing shape,dimension, and/or stiffness of an active material disposed in thedevice.

The above described and other features are exemplified by the followingfigures and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the figures, which are exemplary embodiments andwherein like elements are numbered alike:

FIG. 1 is a block diagram showing common elements of hood mechanisms;

FIG. 2 is an underside plan view of a hood;

FIG. 3 is a schematic view of one embodiment of an active material basedhood pedestrian mitigation device in a compact configuration; and

FIG. 4 is a schematic view of one embodiment of an active material basedhood pedestrian mitigation device in an expanded configuration.

DETAILED DESCRIPTION

Methods, devices and hood assemblies for reversibly increasing theenergy absorption capability at appropriate force levels of a vehiclehood are disclosed herein. As used herein, the term “hood” generallyrefers to the lids covering the engine or trunk areas. The methodgenerally includes sensing the impact, generating a signal, andactivating an active material in operative communication with the hoodupon receipt of the signal. Alternatively, an occupant of the vehiclemay generate the signal manually. The active material, in operativecommunication with the hood, increases the energy absorbing capabilitiesby changing the hood shape, changing the hood stiffness, changing thestiffness of the mounting hardware, and/or changing the hood orientationthrough active lifting and/or active tilting means to provide increasedclearance from underlying engine compartment. The resulting deformationbehavior including stiffness and modulus properties can be alteredeither globally or locally.

In one embodiment, the active material changes the shape or orientationof a vehicle hood in response to an activation signal. A device oractuator contains the active material, wherein the active material has afirst shape, dimension, or stiffness and is operative to change to asecond shape, dimension, stiffness, and/or provide a change in shearstrength in response to the activation signal. The device is designed tobe installed in operative communication with the hood.

In another embodiment, a vehicle system contains an impact sensor thatgenerates an impact signal. The system further contains a controllerdisposed to receive the impact signal and a hood impact mitigationdevice that operates upon receiving the activation signal from thecontroller. The active material changes its shape, stiffness or otherphysical property in response to the activation signal. The mitigationdevice, for example, may be a hood lifter.

In various embodiments, the response of the mitigation device to thesignal may be reversible (to prevent damage in the event that an impactdoes not occur) and/or may be tailored both locally and globally to theparticular nature of the impact event. It may also, for example, in thecase of stiffness changes, be unnoticeable or undetectable (fullyreversible), unless an impact occurs, to the vehicle operator. Further,there is minimal interference with vehicle operation. Common elements tothe various embodiments described herein are illustrated in FIG. 1. Suchelements include a sensor 2 plus a controller 4 for triggering theactive material based mechanism 6. It further contains a power source 8and one or more active materials 14 incorporated into the mechanism 6.In a preferred mode of operation, the mechanism is unpowered duringnormal driving and is activated or powered when triggered by an outputsignal from the controller 4 based on input to it from an impact orpre-impact sensor, schematically illustrated by 9 in FIG. 1. Such amechanism would remain activated through the impact event but thenautomatically be deactivated upon the conclusion of the impact. In analternative embodiment, the mechanism would be deactivated upon a timertiming out, which would be useful in the case of a false detect.

As used herein, the term “active materials” generally refers to thosematerials that exhibit a change in stiffness, dimension, shape, or shearforce upon application of an activation signal. Depending on the activematerial, the activation signal can take the form of an electric field,a temperature change, a magnetic field or a mechanical loading orstressing. Suitable active materials include, without limitation, shapememory alloys (SMA), magnetic shape memory alloys, shape memory polymers(SMP), piezoelectric materials, electroactive polymers (EAP),magnetorheological fluids and elastomers (MR), and electrorheologicalfluids (ER).

Shape memory alloys can exist in several different temperature-dependentphases. The most commonly utilized of these phases are the so-calledmartensite and austenite phases. In the following discussion, themartensite phase generally refers to the more deformable, lowertemperature phase whereas the austenite phase generally refers to themore rigid, higher temperature phase. When the shape memory alloy is inthe martensite phase and is heated, it begins to change into theaustenite phase. The temperature at which this phenomenon starts isoften referred to as austenite start temperature (A_(s)). Thetemperature at which this phenomenon is complete is called the austenitefinish temperature (A_(f)). When the shape memory alloy is in theaustenite phase and is cooled, it begins to change into the martensitephase, and the temperature at which this phenomenon starts is referredto as the martensite start temperature (M_(s)). The temperature at whichshape memory alloy finishes transforming to the martensite phase iscalled the martensite finish temperature (M_(f)). Generally, the shapememory alloys have a lower modulus and are more easily deformable intheir martensitic phase and have a higher modulus and are thus lesseasily deformable in the austenitic phase.

Suitable shape memory alloys can exhibit a one-way shape memory effect,an intrinsic two-way effect, or an extrinsic two-way shape memory effectdepending on the alloy composition and processing history. Preferably, acounterbalancing spring is employed in combination with the shape memoryalloy to provide a restoring force. In this manner, the actuator can bemade reversible. The return spring preferably has a modulus somewherebetween the martensite and austenite phase transformation temperaturesof the shape memory alloy. Optionally, the return spring is apre-existing component of the hood itself that the shape memories alloywas used to deform. Alternatively, separately actuatable parallel SMAelements can be alternately activated, each reversing the action of theother.

Shape memory materials formed from shape memory alloy compositions thatexhibit one-way shape memory effects do not automatically reform, anddepending on the shape memory material design, will likely require anexternal mechanical force to reform the shape, dimension, that waspreviously exhibited. Shape memory materials that exhibit an intrinsicshape memory effect are fabricated from a shape memory alloy compositionthat will automatically reform themselves as a result of the above notedphase transformations.

Intrinsic two-way shape memory behavior is preferably induced in theshape memory material through processing. Such procedures includeextreme deformation of the material while in the martensite phase,heating-cooling under constraint or load, or surface modification suchas laser annealing, polishing, or shot-peening. Once the material hasbeen trained to exhibit the two-way shape memory effect, the shapechange between the low and high temperature states is generallyreversible and persists through a high number of thermal cycles.

Shape memory materials that exhibit the extrinsic two-way shape memoryeffects are composite or multi-component materials that combine a shapememory alloy composition that exhibits a one-way effect with anotherelement that provides a restoring force to reform the original shape.

The temperature at which the shape memory alloy remembers its hightemperature form when heated can be adjusted by slight changes in thecomposition of the alloy and through heat treatment. In nickel-titaniumshape memory alloys, for example, it can be changed from above about100° C. to below about −100° C. The shape recovery process occurs over arange of just a few degrees and the start or finish of thetransformation can be controlled to within a degree or two depending onthe desired application and alloy composition. The mechanical propertiesof the shape memory alloy vary greatly over the temperature rangespanning their transformation, typically providing the active material14 with shape memory effects as well as high damping capacity. Theinherent high damping capacity of the shape memory alloys can be used tofurther increase the energy absorbing properties of the energy absorbingassembly.

Suitable shape memory alloy materials include without limitationnickel-titanium based alloys, indium-titanium based alloys,nickel-aluminum based alloys, nickel-gallium based alloys, copper basedalloys (e.g., copper-zinc alloys, copper-aluminum alloys, copper-gold,and copper-tin alloys), gold-cadmium based alloys, silver-cadmium basedalloys, indium-cadmium based alloys, manganese-copper based alloys,iron-platinum based alloys, iron-platinum based alloys, iron-palladiumbased alloys, and the like. The alloys can be binary, ternary, or anyhigher order so long as the alloy composition exhibits a shape memoryeffect, e.g., change in shape, dimension, damping capacity, and thelike. For example, a nickel-titanium based alloy is commerciallyavailable under the trademark NITINOL from Shape Memory Applications,Inc.

Other suitable active materials are shape memory polymers. Similar tothe behavior of a shape memory alloy, when the temperature is raisedthrough its transition temperature, the shape memory polymer alsoundergoes a change in shape, dimension. To set the permanent shape ofthe shape memory polymer, the polymer must be at about or above the Tgor melting point of the hard segment of the polymer. “Segment” refers toa block or sequence of polymer forming part of the shape memory polymer.The shape memory polymers are shaped at the temperature with an appliedforce followed by cooling to set the permanent shape. The temperaturenecessary to set the permanent shape is preferably between about 100° C.to about 300° C. Setting the temporary shape of the shape memory polymerrequires the shape memory polymer material to be brought to atemperature at or above the Tg or transition temperature of the softsegment, but below the Tg or melting point of the hard segment. At thesoft segment transition temperature (also termed “first transitiontemperature”), the temporary shape of the shape memory polymer is setfollowed by cooling of the shape memory polymer to lock in the temporaryshape. The temporary shape is maintained as long as it remains below thesoft segment transition temperature. The permanent shape is regainedwhen the shape memory polymer fibers are once again brought to or abovethe transition temperature of the soft segment. Repeating the heating,shaping, and cooling steps can reset the temporary shape. The softsegment transition temperature can be chosen for a particularapplication by modifying the structure and composition of the polymer.Transition temperatures of the soft segment range from about −63° C. toabove about 120° C.

Shape memory polymers may contain more than two transition temperatures.A shape memory polymer composition comprising a hard segment and twosoft segments can have three transition temperatures: the highesttransition temperature for the hard segment and a transition temperaturefor each soft segment.

Most shape memory polymers exhibit a “one-way” effect, wherein the shapememory polymer exhibits one permanent shape. Upon heating the shapememory polymer above the first transition temperature, the permanentshape is achieved and the shape will not revert back to the temporaryshape without the use of outside forces. As an alternative, some shapememory polymer compositions can be prepared to exhibit a “two-way”effect. These systems consist of at least two polymer components. Forexample, one component could be a first cross-linked polymer while theother component is a different cross-linked polymer. The components arecombined by layer techniques, or are interpenetrating networks, whereintwo components are cross-linked but not to each other. By changing thetemperature, the shape memory polymer changes its shape in the directionof the first permanent shape to the second permanent shape. Each of thepermanent shapes belongs to one component of the shape memory polymer.The two permanent shapes are always in equilibrium between both shapes.The temperature dependence of the shape is caused by the fact that themechanical properties of one component (“component A”) are almostindependent from the temperature in the temperature interval ofinterest. The mechanical properties of the other component (“componentB”) depend on the temperature. In one embodiment, component B becomesstronger at low temperatures compared to component A, while component Ais stronger at high temperatures and determines the actual shape. Atwo-way memory device can be prepared by setting the permanent shape ofcomponent A (“first permanent shape”); deforming the device into thepermanent shape of component B (“second permanent shape”) and fixing thepermanent shape of component B while applying a stress to the component.

Similar to the shape memory alloy materials, the shape memory polymerscan be configured in many different forms and shapes. The temperatureneeded for permanent shape recovery can be set at any temperaturebetween about −63° C. and about 120° C. or above. Engineering thecomposition and structure of the polymer itself can allow for the choiceof a particular temperature for a desired application. A preferredtemperature for shape recovery is greater than or equal to about −30°C., more preferably greater than or equal to about 0° C., and mostpreferably a temperature greater than or equal to about 50° C. Also, apreferred temperature for shape recovery is less than or equal to about120° C., more preferably less than or equal to about 90° C., and mostpreferably less than or equal to about 70° C.

Suitable shape memory polymers include thermoplastics, thermosets,interpenetrating networks, semi-interpenetrating networks, or mixednetworks. The polymers can be a single polymer or a blend of polymers.The polymers can be linear or branched thermoplastic elastomers withside chains or dendritic structural elements. Suitable polymercomponents to form a shape memory polymer include, but are not limitedto, polyphosphazenes, poly(vinyl alcohols), polyamides, polyesteramides, poly(amino acid)s, polyanhydrides, polycarbonates,polyacrylates, polyalkylenes, polyacrylamides, polyalkylene glycols,polyalkylene oxides, polyalkylene terephthalates, polyortho esters,polyvinyl ethers, polyvinyl esters, polyvinyl halides, polyesters,polylactides, polyglycolides, polysiloxanes, polyurethanes, polyethers,polyether amides, polyether esters, and copolymers thereof. Examples ofsuitable polyacrylates include poly(methyl methacrylate), poly(ethylmethacrylate), ply(butyl methacrylate), poly(isobutyl methacrylate),poly(hexyl methacrylate), poly(isodecyl methacrylate), poly(laurylmethacrylate), poly(phenyl methacrylate), poly(methyl acrylate),poly(isopropyl acrylate), poly(isobutyl acrylate) and poly(octadecylacrylate). Examples of other suitable polymers include polystyrene,polypropylene, polyvinyl phenol, polyvinylpyrrolidone, chlorinatedpolybutylene, poly(octadecyl vinyl ether) ethylene vinyl acetate,polyethylene, poly(ethylene oxide)-poly(ethylene terephthalate),polyethylene/nylon (graft copolymer), polycaprolactones-polyamide (blockcopolymer), poly(caprolactone) dimethacrylate-n-butyl acrylate,poly(norbornyl-polyhedral oligomeric silsequioxane), polyvinylchloride,urethane/butadiene copolymers, polyurethane block copolymers,styrene-butadiene-styrene block copolymers, and the like.

The shape memory polymer or the shape memory alloy, may be activated byany suitable means, preferably a means for subjecting the material to atemperature change above, or below, a transition temperature. Forexample, for elevated temperatures, heat may be supplied using hot gas(e.g., air), steam, hot liquid, or electrical current. The activationmeans may, for example, be in the form of heat conduction from a heatedelement in contact with the shape memory material, heat convection froma heated conduit in proximity to the thermally active shape memorymaterial, a hot air blower or jet, microwave interaction, resistiveheating, and the like. In the case of a temperature drop, heat may beextracted by using cold gas, or evaporation of a refrigerant. Theactivation means may, for example, be in the form of a cool room orenclosure, a cooling probe having a cooled tip, a control signal to athermoelectric unit, a cold air blower or jet, or means for introducinga refrigerant (such as liquid nitrogen) to at least the vicinity of theshape memory material.

Suitable magnetic materials include, but are not intended to be limitedto, soft or hard magnets; hematite; magnetite; magnetic material basedon iron, nickel, and cobalt, alloys of the foregoing, or combinationscomprising at least one of the foregoing, and the like. Alloys of iron,nickel and/or cobalt, can comprise aluminum, silicon, cobalt, nickel,vanadium, molybdenum, chromium, tungsten, manganese and/or copper.

Suitable MR fluid materials include, but are not intended to be limitedto, ferromagnetic or paramagnetic particles dispersed in a carrierfluid. Suitable particles include iron; iron alloys, such as thoseincluding aluminum, silicon, cobalt, nickel, vanadium, molybdenum,chromium, tungsten, manganese and/or copper; iron oxides, includingFe₂O₃ and Fe₃O₄; iron nitride; iron carbide; carbonyl iron; nickel andalloys of nickel; cobalt and alloys of cobalt; chromium dioxide;stainless steel; silicon steel; and the like. Examples of suitableparticles include straight iron powders, reduced iron powders, ironoxide powder/straight iron powder mixtures and iron oxide powder/reducediron powder mixtures. A preferred magnetic-responsive particulate iscarbonyl iron, preferably, reduced carbonyl iron.

The particle size should be selected so that the particles exhibitmulti-domain characteristics when subjected to a magnetic field.Diameter sizes for the particles can be less than or equal to about 1000micrometers, with less than or equal to about 500 micrometers preferred,and less than or equal to about 100 micrometers more preferred. Alsopreferred is a particle diameter of greater than or equal to about 0.1micrometer, with greater than or equal to about 0.5 more preferred, andgreater than or equal to about 10 micrometers especially preferred. Theparticles are preferably present in an amount between about 5.0 to about50 percent by volume of the total MR fluid composition.

Suitable carrier fluids include organic liquids, especially non-polarorganic liquids. Examples include, but are not limited to, siliconeoils; mineral oils; paraffin oils; silicone copolymers; white oils;hydraulic oils; transformer oils; halogenated organic liquids, such aschlorinated hydrocarbons, halogenated paraffins, perfluorinatedpolyethers and fluorinated hydrocarbons; diesters; polyoxyalkylenes;fluorinated silicones; cyanoalkyl siloxanes; glycols; synthetichydrocarbon oils, including both unsaturated and saturated; andcombinations comprising at least one of the foregoing fluids.

The viscosity of the carrier component can be less than or equal toabout 100,000 centipoise, with less than or equal to about 10,000centipoise preferred, and less than or equal to about 1,000 centipoisemore preferred. Also preferred is a viscosity of greater than or equalto about 1 centipoise, with greater than or equal to about 250centipoise preferred, and greater than or equal to about 500 centipoiseespecially preferred.

Aqueous carrier fluids may also be used, especially those comprisinghydrophilic mineral clays such as bentonite or hectorite. The aqueouscarrier fluid may comprise water or water comprising a small amount ofpolar, water-miscible organic solvents such as methanol, ethanol,propanol, dimethyl sulfoxide, dimethyl formamide, ethylene carbonate,propylene carbonate, acetone, tetrahydrofuran, diethyl ether, ethyleneglycol, propylene glycol, and the like. The amount of polar organicsolvents is less than or equal to about 5.0% by volume of the total MRfluid, and preferably less than or equal to about 3.0%. Also, the amountof polar organic solvents is preferably greater than or equal to about0.1%, and more preferably greater than or equal to about 1.0% by volumeof the total MR fluid. The pH of the aqueous carrier fluid is preferablyless than or equal to about 13, and preferably less than or equal toabout 9.0. Also, the pH of the aqueous carrier fluid is greater than orequal to about 5.0, and preferably greater than or equal to about 8.0.

Natural or synthetic bentonite or hectorite may be used. The amount ofbentonite or hectorite in the MR fluid is less than or equal to about 10percent by weight of the total MR fluid, preferably less than or equalto about 8.0 percent by weight, and more preferably less than or equalto about 6.0 percent by weight. Preferably, the bentonite or hectoriteis present in greater than or equal to about 0.1 percent by weight, morepreferably greater than or equal to about 1.0 percent by weight, andespecially preferred greater than or equal to about 2.0 percent byweight of the total MR fluid.

Optional components in the MR fluid include clays, organoclays,carboxylate soaps, dispersants, corrosion inhibitors, lubricants,extreme pressure anti-wear additives, antioxidants, thixotropic agentsand conventional suspension agents. Carboxylate soaps include ferrousoleate, ferrous naphthenate, ferrous stearate, aluminum di- andtri-stearate, lithium stearate, calcium stearate, zinc stearate andsodium stearate, and surfactants such as sulfonates, phosphate esters,stearic acid, glycerol monooleate, sorbitan sesquioleate, laurates,fatty acids, fatty alcohols, fluoroaliphatic polymeric esters, andtitanate, aluminate and zirconate coupling agents and the like.Polyalkylene diols, such as polyethylene glycol, and partiallyesterified polyols can also be included.

Suitable MR elastomer materials include, but are not intended to belimited to, an elastic polymer matrix comprising a suspension offerromagnetic or paramagnetic particles, wherein the particles aredescribed above. Suitable polymer matrices include, but are not limitedto, poly-alpha-olefins, natural rubber, silicone, polybutadiene,polyethylene, polyisoprene, and the like.

Electroactive polymers include those polymeric materials that exhibitpiezoelectric, pyroelectric, or electrostrictive properties in responseto electrical or mechanical fields. The materials generally employ theuse of compliant electrodes that enable polymer films to expand orcontract in the in-plane directions in response to applied electricfields or mechanical stresses. An example of an electrostrictive-graftedelastomer with a piezoelectric poly(vinylidenefluoride-trifluoro-ethylene) copolymer. This combination has the abilityto produce a varied amount of ferroelectric-electrostrictive molecularcomposite systems. These may be operated as a piezoelectric sensor oreven an electrostrictive actuator.

Materials suitable for use as an electroactive polymer may include anysubstantially insulating polymer or rubber (or combination thereof) thatdeforms in response to an electrostatic force or whose deformationresults in a change in electric field. Exemplary materials suitable foruse as a pre-strained polymer include silicone elastomers, acrylicelastomers, polyurethanes, thermoplastic elastomers, copolymerscomprising PVDF, pressure-sensitive adhesives, fluoroelastomers,polymers comprising silicone and acrylic moieties, and the like.Polymers comprising silicone and acrylic moieties may include copolymerscomprising silicone and acrylic moieties, polymer blends comprising asilicone elastomer and an acrylic elastomer, for example.

Materials used as an electroactive polymer may be selected based on oneor more material properties such as a high electrical breakdownstrength, a low modulus of elasticity—(for large or small deformations),a high dielectric constant, and the like. In one embodiment, the polymeris selected such that is has an elastic modulus at most about 100 MPa.In another embodiment, the polymer is selected such that is has amaximum actuation pressure between about 0.05 MPa and about 10 MPa, andpreferably between about 0.3 MPa and about 3 MPa. In another embodiment,the polymer is selected such that is has a dielectric constant betweenabout 2 and about 20, and preferably between about 2.5 and about 12. Thepresent disclosure is not intended to be limited to these ranges.Ideally, materials with a higher dielectric constant than the rangesgiven above would be desirable if the materials had both a highdielectric constant and a high dielectric strength. In many cases,electroactive polymers may be fabricated and implemented as thin films.Thicknesses suitable for these thin films may be below 50 micrometers.

As electroactive polymers may deflect at high strains, electrodesattached to the polymers should also deflect without compromisingmechanical or electrical performance. Generally, electrodes suitable foruse may be of any shape and material provided that they are able tosupply a suitable voltage to, or receive a suitable voltage from, anelectroactive polymer. The voltage may be either constant or varyingover time. In one embodiment, the electrodes adhere to a surface of thepolymer. Electrodes adhering to the polymer are preferably compliant andconform to the changing shape of the polymer. Correspondingly, thepresent disclosure may include compliant electrodes that conform to theshape of an electroactive polymer to which they are attached. Theelectrodes may be only applied to a portion of an electroactive polymerand define an active area according to their geometry. Various types ofelectrodes suitable for use with the present disclosure includestructured electrodes comprising metal traces and charge distributionlayers, textured electrodes comprising varying out of plane dimensions,conductive greases such as carbon greases or silver greases, colloidalsuspensions, high aspect ratio conductive materials such as carbonfibrils and carbon nanotubes, and mixtures of ionically conductivematerials.

Materials used for electrodes of the present disclosure may vary.Suitable materials used in an electrode may include graphite, carbonblack, colloidal suspensions, thin metals including silver and gold,silver filled and carbon filled gels and polymers, and ionically orelectronically conductive polymers. It is understood that certainelectrode materials may work well with particular polymers and may notwork as well for others. By way of example, carbon fibrils work wellwith acrylic elastomer polymers while not as well with siliconepolymers.

The active material may also comprise a piezoelectric material. Also, incertain embodiments, the piezoelectric material may be configured as anactuator for providing rapid deployment. As used herein, the term“piezoelectric” is used to describe a material that mechanically deforms(changes shape) when a voltage potential is applied, or conversely,generates an electrical charge when mechanically deformed. Preferably, apiezoelectric material is disposed on strips of a flexible metal orceramic sheet. The strips can be unimorph or bimorph. Preferably, thestrips are bimorph, because bimorphs generally exhibit more displacementthan unimorphs.

One type of unimorph is a structure composed of a single piezoelectricelement externally bonded to a flexible metal foil or strip, which isstimulated by the piezoelectric element when activated with a changingvoltage and results in an axial buckling or deflection as it opposes themovement of the piezoelectric element. The actuator movement for aunimorph can be by contraction or expansion. Unimorphs can exhibit astrain of as high as about 10%, but generally can only sustain low loadsrelative to the overall dimensions of the unimorph structure. Acommercial example of a pre-stressed unimorph is referred to as“THUNDER”, which is an acronym for THin layer composite UNimorphferroelectric Driver and sEnsoR. THUNDER is a composite structureconstructed with a piezoelectric ceramic layer (for example, leadzirconate titanate), which is electroplated on its two major faces. Ametal pre-stress layer is adhered to the electroplated surface on atleast one side of the ceramic layer by an adhesive layer (for example,“LaRC-SI®” developed by the National Aeronautics and SpaceAdministration (NASA)). During manufacture of a THUNDER actuator, theceramic layer, the adhesive layer, and the first pre-stress layer aresimultaneously heated to a temperature above the melting point of theadhesive, and then subsequently allowed to cool, thereby re-solidifyingand setting the adhesive layer. During the cooling process the ceramiclayer becomes strained, due to the higher coefficients of thermalcontraction of the metal pre-stress layer and the adhesive layer than ofthe ceramic layer. Also, due to the greater thermal contraction of thelaminate materials than the ceramic layer, the ceramic layer deformsinto an arcuate shape having a generally concave face.

In contrast to the unimorph piezoelectric device, a bimorph deviceincludes an intermediate flexible metal foil sandwiched between twopiezoelectric elements. Bimorphs exhibit more displacement thanunimorphs because under the applied voltage one ceramic element willcontract while the other expands. Bimorphs can exhibit strains up toabout 20%, but similar to unimorphs, generally cannot sustain high loadsrelative to the overall dimensions of the unimorph structure.

Suitable piezoelectric materials include inorganic compounds, organiccompounds, and metals. With regard to organic materials, all of thepolymeric materials with non-centrosymmetric structure and large dipolemoment group(s) on the main chain or on the side-chain, or on bothchains within the molecules, can be used as candidates for thepiezoelectric film. Examples of suitable polymers include, for example,but are not limited to, poly(sodium 4-styrenesulfonate) (“PSS”), polyS-119 (poly(vinylamine)backbone azo chromophore), and their derivatives;polyfluorocarbons, including polyvinylidene fluoride (“PVDF”), itsco-polymer vinylidene fluoride (“VDF”), trifluoroethylene (TrFE), andtheir derivatives; polychlorocarbons, including poly(vinyl chloride)(“PVC”), polyvinylidene chloride (“PVC2”), and their derivatives;polyacrylonitriles (“PAN”), and their derivatives; polycarboxylic acids,including poly(methacrylic acid (“PMA”), and their derivatives;polyureas, and their derivatives; polyurethanes (“PUE”), and theirderivatives; bio-polymer molecules such as poly-L-lactic acids and theirderivatives, and membrane proteins, as well as phosphate bio-molecules;polyanilines and their derivatives, and all of the derivatives oftetramines; polyimides, including Kapton molecules and polyetherimide(“PEI”), and their derivatives; all of the membrane polymers;poly(N-vinyl pyrrolidone) (“PVP”) homopolymer, and its derivatives, andrandom PVP-co-vinyl acetate (“PVAc”) copolymers; and all of the aromaticpolymers with dipole moment groups in the main-chain or side-chains, orin both the main-chain and the side-chains, and mixtures thereof.

Further, piezoelectric materials can include Pt, Pd, Ni, Ti, Cr, Fe, Ag,Au, Cu, and metal alloys and mixtures thereof. These piezoelectricmaterials can also include, for example, metal oxide such as SiO₂,Al₂O₃, ZrO₂, TiO₂, SrTiO₃, PbTiO₃, BaTiO₃, FeO₃, Fe₃O₄, ZnO, andmixtures thereof; and Group VIA and IIB compounds, such as CdSe, CdS,GaAs, AgCaSe 2, ZnSe, GaP, InP, ZnS, and mixtures thereof.

The action of the active material in the impact mitigation mechanism maybe used either directly or indirectly to either reversibly orirreversibly raise or tilt the hood globally, deform (change the shapeof the hood) globally, raise (deform) the hood locally, change the forceneeded to locally deform the hood (by for example providing a localchange in material stiffness) and change the applied load needed toglobally displace the hood (for example by changing the stroking forcein ER and MR material hood mounts, attachments or lifters or by changingthe stiffness of supporting or lifting springs made of shape memoryalloys, and the like).

In some embodiments, the functionality is not provided entirely by theactive material. In general, an active material is used to provide atleast one, but not necessarily all of the following functions: changesin stiffness, actuation (changes in dimension, and/or shape eitherlocally or globally of the hood and hood tilting or displacement),impact energy absorption and the tailorability thereof, and aself-healing or reversibility of the mechanism.

One embodiment of an active material based reversibly expandable ondemand pad to locally raise the hood above a hard item beneath it isshown in FIG. 2. Such a pad may be mounted to the underside of the hoodjust above the hard object such that the on demand expansion of the padwould first fill any space between the hood and hard object (in thismanner changing the compliance of the hood), and then deform the hood(if additional energy absorption capability is desired) upward uponfurther expansion. In FIG. 2, there is shown an underside of anexemplary hood, generally designated by reference numeral 10. As shown,active material based pads 12 are positioned about an underside of thehood 10. The exact positioning of the pads will depend on the energyabsorption properties desired for the intended application. Althoughreference is made to the underside of the hood 10, it is contemplatedthat the active based material pads could be attached to the vehiclerails upon which the hood rests and is hinged thereto.

As shown in FIGS. 3 and 4, the active material based pads 12 generallyinclude a covering 13 and an active material 14 in operativecommunication with the covering 13. Figure 3 illustrates the assembly ina compacted configuration and FIG. 4 illustrates the pad assembly in anexpanded configuration. As shown, the active material based pad 12 isprovided with a lower plate 16 adapted to provide mechanicalcommunication with under hood components 25 such as an engine block andwith an upper plate 17 that communicates with the underside of the hood10. The pad may be installed by attaching either to the hood or to anunder hood component, e.g., rails. As installed, the assembly at leastpartially fills the space between the hood 10 and components beneath thehood 10, e.g., engine. In one embodiment, the active material 14 isadapted to change shape to effect a change in the length dimension andcause the covering 13 to expand and/or detach in response to anactivation signal. Alternatively, a pyrotechnic device or a storedelastic energy source can be employed to raise the hood 20, wherein theactive material based pad 12 provides a mechanism for restoring the hoodto its original position. In other embodiments, the active materialbased pads 12 can be configured to provide both expansion andcontraction forces. That is, a portion of the pads can be adapted toprovide an expansive force upon deployment whereas another portion canbe adapted to provide contractive forces upon deployment.

In some embodiments, upon discontinuation of the activation signal, theactive material can change substantially back to its original shape (andoriginal length dimension) and simultaneously contract the coveringsubstantially back to its original shape and/or original position. Theactivation signal provided for changing the shape, dimension, of theactive material 14 may include a heat signal, an electrical signal, apneumatic signal, a mechanical activation signal, combinationscomprising at least one of the foregoing signals, and the like; theparticular activation signal depending on the materials and/orconfiguration of the shape memory material 14. In some embodiments, theactive material 14 thermally increases its length dimension in responseto the activation signal to cause the covering 13 to expand and/ordetach from its surrounding surface medium. At the same time, expansionof the pad causes plates 16 and 17 to engage the under hood componentand hood respectively so as to deform the hood upward upon furtherexpansion. The hood and the pad 10, in its expanded form can then beused to absorb the kinetic energy of an object hitting the hood. Thecovering 13 and the shape memory material 14, individually as well as incombination, may provide additional energy absorbing properties for theactive material based pad 12.

In another embodiment, the active material based pad 12 furthercomprises a sensor 18 and a controller 20 in operative communicationwith the active material 14 for expanding (and/or detaching) thecovering 13 in response to an activation signal provided by the sensor18. The sensor 18 is preferably configured to provide pre-impactinformation to the controller 20, which then actuates the hood liftingactive material based pad 12 under pre-programmed conditions defined byan algorithm or the like. In this manner, the active material based pad12 can be used to anticipate an event such as an impact with an objectand provide a change in the hood configuration before absorption of thekinetic energy associated with the object as a result of the impact. Theactive material based pad 12 is exemplary only and is not intended to belimited to any particular shape, size, configuration, or the like.

The active material 14 preferably comprises a material that can beactivated to provide expansion and/or detachment of the covering 13. Insome embodiments, the active material 14 is chosen to providecontraction capabilities as well. In this manner, the active materialbased pad 12 is reversible and repeated use is available for example insituations where damage from impact is only very slight, or where impactis avoided altogether after actuation of the hood-lifting pad. Aspreviously disclosed, preferred active materials 14 include shape memoryalloys, electroactive polymers, shape memory polymers, piezoelectrics,and the like.

In some embodiments, the active materials comprise shape memory alloysin the form of fibers. The fibers may be configured as springs, loops,interconnecting networks, and the like. The fibers may be formedintegrally with the support structure, or more preferably, may beattached directly to the lower plate support structure 16 and/or thecovering 13. For example, the shape memory alloys can be mechanicallyclamped to the plate, an adhesive can be applied (e.g., silver-dopedepoxy) to the lower plate support structure 16 and/or the covering 13and the active material 14 can be mechanically pressed into theadhesive, and the like attachment means. Alternatively, vapor grownshape memory alloy fibers can be deposited directly from a gas phaseonto a lower plate 16 and/or the covering 13. Preferably, the thickness,length, and overall geometry of the shape memory alloy fiber is suitablefor providing an effective length dimension change at sufficient forcelevels to achieve actuation of hood lifting, i.e., to cause the covering13 to expand and/or detach from the surrounding surface medium. Theshape memory alloy fiber should also be of a thickness, length, andoverall geometry effective to provide the desired shape memory effect.The fibers are not intended to be limited to any particular shape.

The lower plate supporting structure 16 may also comprise the activationdevice for providing the thermal activating signal to shape memorymaterial depending on the particular design of the energy absorbingassembly. For example, the lower plate supporting structure 16 mayincorporate a resistance type-heating block to provide a thermal energysignal sufficient to cause the shape change.

Employing the piezoelectric material will utilize an electrical signalfor activation. Upon activation, the piezoelectric material will assumean arcuate shape, thereby causing displacement in the powered state.Upon discontinuation of the activation signal, the strips will assumeits original shape, dimension, e.g., a straightened shape, dimension.

Similarly, activation of an EAP based pad preferably utilizes anelectrical signal to provide change in shape, dimension, sufficient toprovide displacement. Reversing the polarity of the applied voltage tothe EAP can provide a reversible mechanism.

As previously discussed, the various shapes of the active material 14employed in the energy absorbing active material based pads 12 arevirtually limitless. Suitable geometrical arrangements may includecellular metal textiles, open cell foam structures, multiple layers ofshape memory material similar to “bubble wrap”, arrays of hooks and/orloops, and the like.

The activation times will generally vary depending on the intendedapplication, the particular active material employed, the magnitude ofthe activation signal, and the like. For example, for hood and trunklock downs, it is generally preferred to have an activation time of lessthan about 10 milliseconds, an activation time of less than 5milliseconds more preferred for some applications, an activation time ofless than 3 milliseconds even more preferred for other applications, andan activation time of less than 0.5 milliseconds for still otherapplications.

Advantageously, the hood assemblies utilizing the active materials toeffect changes in energy absorption properties provides a relativelyrobust system compared to conventional systems utilizing strokingmechanisms based on hydraulics, and the like. Moreover, in addition toproviding reversibility, the active material based actuators arerelatively compact and of significantly lower weight. It should berecognized by those skilled in the art that the active materials as usedherein allows the use of pre-crash sensors.

While the disclosure has been described with reference to an exemplaryembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the disclosure. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the disclosure without departing fromthe essential scope thereof. Therefore, it is intended that thedisclosure not be limited to the particular embodiment disclosed as thebest mode contemplated for carrying out this disclosure, but that thedisclosure will include all embodiments falling within the scope of theappended claims.

1. A vehicle, comprising: a hood; an underhood component; an activematerial based pad including a lower plate in mechanical communicationwith the underhood component, an upper plate in mechanical communicationwith the hood, and an active material between the upper and lowerplates, wherein the active material is configured to increase in lengthin response to an activation signal, thereby to urge the upper and lowerplates apart; and an activation device coupled to the active material.2. A vehicle comprising: a hood; an underhood component; active materialoperative to increase in length thereby to increase the distance betweenthe hood and the underhood component in response to an activationsignal.
 3. The vehicle of claim 2, wherein the active material comprisesa shape memory alloy, a magnetic shape memory alloy, a shape memorypolymer, an electroactive polymer, or a piezoelectric material.
 4. Thevehicle of claim 2, wherein the activation signal comprises a thermalactivation signal, a magnetic activation signal, an electric activationsignal, a chemical activation signal, or a mechanical load.
 5. Thevehicle of claim 2, wherein the active material undergoes a change inshape when subjected to the activation signal, and wherein the change inshape is reversible.
 6. A system for mitigating damage to an object fromimpact with a vehicle hood, comprising: a sensor that generates a signalbased on pre-impact or impact information; and a controller disposed toreceive the sensor signal and deliver an activation signal to at leastone device in operative communication with the hood and an underhoodcomponent; wherein the at least one device includes a lower plate, anupper plate, and an active material therebetween, wherein the activematerial is operative to increase from a first length to a second lengthin response to the activation signal.
 7. The system according to claim6, wherein the lower plate is in contact with the underhood componentand the upper plate in contact with an underside of the hood, andwherein the change from the first length to the second length iseffective to increase a space between the vehicle and the hood.
 8. Thesystem according to claim 6, wherein the active material comprises ashape memory alloy, a shape memory polymer, an electroactive polymer, ora piezoelectric material.
 9. The system according to claim 6, whereinthe increase from the first length to the second length in response tothe activation signal changes an applied load to displace the hood andincreases the energy absorbing properties of the hood.
 10. The systemaccording to claim 6, wherein the increase from the first length to thesecond length in response to the activation signal changes a forceneeded to locally deform the hood.
 11. A method for reducing damage toan object from an impact with a hood, comprising: sensing the impact;generating an activation signal; and activating a device in response tothe activation signal, wherein the device is disposed between anunderside of the hood and an underhood component and comprises an activematerial based pad having a lower plate, an upper plate, and an activematerial therebetween, wherein the active material changes a position ofthe hood relative to the underhood component from a first position to asecond position by actively increasing a length dimension of the activematerial disposed in the device.
 12. The method of claim 11, whereinchanging the position of the hood from the first position to the secondposition is reversible.
 13. The method of claim 11, wherein the activematerial comprises a shape memory alloy, a magnetic shape memory alloy,a shape memory polymer, an electroactive polymer, or a piezoelectricmaterial.
 14. A method according to claim 11, wherein sensing isaccomplished with a pre-impact sensor.
 15. A method according to claim11, wherein sensing is accomplished with an impact sensor.
 16. A methodaccording to claim 11, wherein the generating the activation signalcomprises manual activation.
 17. A method according to claim 11, whereinthe object is a pedestrian.